Why does the 155s run on 225s when the 156s run on 205s...they both have the same offset, diameter, width ...??

Here in Canada the 155's are included in the Sport package only on the entry level 323i which has a 2.5 litre 170hp inline 6.
The tires are the 225/50R16 performance RE050 runflats.
Also on the 323i without Sport package we get a style 154 16x7 double 5 spoke alloy that is not overly appealing. (they look small)
Like the 156's they roll on 205/55R16 all season EL42 runflats. (also not very appealing)
Mind you none of the runflats appeal to me, I have gone through the poor ride quality issue with runflats on my MINI.